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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Postbuckling and failure characteristics of selected flat rectangular graphite-epoxy plates loaded in compression

Results of an experimental study of the postbuckling response and failure characteristics of 16- and 24-ply quasi-isotropic and 24-ply orthotropic flat rectangular graphite-epoxy plates loaded in compression are examined. Some of the specimens had circular drilled holes or were subjected to low-speed impact damage. The ratio of failure load to buckling load was found to be higher for specimens with lower initial buckling strains than for specimens with higher initial buckling strains. Some specimens supported more than five times their initial buckling load before failing. The specimens failed along a nodal line of the buckling mode in an induced shear failure mode. Holes and impact damage had a more adverse effect on the postbuckling strength of specimens with high initial buckling strains than on the strength of specimens with lower initial buckling strains.

Starnes, J. H., Jr.↗
NASA NTRS2018

Transition Induced by Tandem Rectangular Roughness Elements on a Supersonic Flat Plate

The flow behind two rectangular roughness elements with a height approximately 38-41 percent of the boundary layer thickness was examined with a hot-wire probe. The rectangular roughness elements are oriented so that one element was at a plus 45-degree angle relative to the leading edge of the plate. A second roughness element was placed 7.16 millimeters downstream of the first one with either the same orientation relative to the leading edge of the plate, or an opposing orientation of minus 45 degrees from the leading edge. Mean mass-flux and total-temperature profiles of the flow field downstream of the tandem roughness elements were examined for mean-flow distortion. Using streak strength as a measure of mean-flow distortion, the tandem roughness elements had approximately the same amount of distortion, regardless of their relative orientation. Mass-flux fluctuation profiles show that the dominant mode downstream of the tandem roughness elements with the same orientation was similar to that of a single roughness element and centered at a frequency of approximately 55 kilohertz (kHz). The dominant instability downstream of the tandem roughness elements with opposing orientation was centered at a frequency of 65 kHz and grew more slowly than the instabilities behind the single roughness element.

Chou, Amanda↗
NASA NTRS1995

Safety analysis

We are engaged in a research program in safety-critical computing that is based on two case studies. We use these case studies to provide application-specific details of the various research issues, and as targets for evaluation of research ideas. The first case study is the Magnetic Stereotaxis System (MSS), an investigational device for performing human neurosurgery being developed in a joint effort between the Department of Physics at the University of Virginia and the Department of Neurosurgery at the University of Iowa. The system operates by manipulating a small permanent magnet (known as a 'seed') within the brain using an externally applied magnetic field. By varying the magnitude and gradient of the external magnetic field, the seed can be moved along a non-linear path and positioned at a site requiring therapy, e.g., a tumor. The magnetic field required for movement through brain tissue is extremely high, and is generated by a set of six superconducting magnets located in a housing surrounding the patient's head. The system uses two X-ray cameras positioned at right angles to detect in real time the locations of the seed and of X-ray opaque markers affixed to the patient's skull. the X-ray images are used to locate the objects of interest in a canonical frame of reference. the second case study is the University of Virginia Research Nuclear Reactor (UVAR). It is a 2 MW thermal, concrete-walled pool reactor. The system operates using 20 to 25 plate-type fuel assemblies placed on a rectangular grid plate. There are three scramable safety rods, and one non-scramable regulating rod that can be put in automatic mode. It was originally constructed in 1959 as a 1 MW system, and it was upgraded to 2 MW in 1973. Though only a research reactor rather than a power reactor, the issues raised are significant and can be related to the problems faced by full-scale reactor systems.

Knight, John C.↗
DOE OSTI2022

Imposition of local boundary conditions in peridynamics without a fictitious layer and unphysical stress concentrations

Here, this study introduces a general approach for the imposition of local boundary conditions in non- ordinary state-based peridynamics (NOSB PD) to eliminate the displacement kinks near the boundary without a fictitious layer under quasi-static loading conditions. It identifies the underlying reason for the unphysical displacement kinks. Under an imposed linear displacement field, the NOSB PD equilibrium equation is not satisfied near the boundary due to the unsymmetric horizon of material points. However, the equilibrium equation derived by using the PD differential operator is satisfied at such material points. Therefore, the material body is divided into three regions to satisfy the equilibrium equations and to impose displacement and tractions boundary conditions. This approach does not deviate from the original NOSB PD; however, it provides a simple solution to eliminate the displacement kink near the boundary, which leads to unphysical stress concentrations. Its efficacy is demonstrated by considering elastic rectangular and square plates subjected to various types of boundary conditions leading to homogeneous as well as nonhomogeneous deformations. The creep response of a rectangular plate further proves the robustness of the present approach. Also, a quasi-static crack propagation from a pre-existing crack in a square plate under mode-I, mode-II and mixed-mode loading conditions demonstrates its capability for failure prediction based on the critical stretch criteria. Finally, its applicability for 3D analysis is demonstrated by considering a rectangular prism under applied stretch and normal stress.

42 ENGINEERING↗
NASA NTRS1991

Postbuckling response and failure prediction of flat rectangular graphite-epoxy plates loaded in axial compression

The objectives of the study are to assess the capability of a first-order shear deformable degenerated shell finite element theory to predict the postbuckling response and failure modes of various graphite-epoxy panels loaded in axial compression. Two panels without holes and one with a hole are studied, and the resulting responses and failure modes correlated well with the experimental results. A progressive damage failure mechanism is applied in the nonlinear analysis, which proved successful in predictions of failure location, mode, and load.

Engelstad, S. P.↗